Control system



y 1957 R. D. MOHLER ET AL CONTROL SYSTEM 2 Sheets-Sheet 2 Filed May 9,1955 WEEK United States Patent CONTROL SYSTEM Robert D. Mohler,Elmhurst, and Walter A. Kelley, Skokie, Ill., assignors to Motorola,Inc., Chicago, Ill., a corporation of Illinois Application May 9, 1955,Serial No. 507,116

11 Claims. (Cl. 317-149) This invention relates generally to remotecontrol systems, and more particularly to systems wherein controlsignals are transmitted by frequency shift of voice frequency carriersand received and utilized for high speed control, with the controloperation being blocked when the signals are not substantially greaterthan noise.

In many applications, it is desired to transfer a control from one pointto another, and to reduce the cost of such systems a plurality ofcontrols may be applied over a single channel. When it is desired toprovide fast operation, the channels must have substantial band widthand, accordingly, the number will be reduced. Although in someapplications it is merely necessary to produce an indication of thecondition of equipment and speed is not particularly important, in otherapplications it is desired to directly control equipment and to be surethat the proper control takes place accurately and rapidly.

As an example of such remote control systems, in power line operation abreaker at a particularpoint may open because of conditions existingalong the line. It may be desired to open other breakers when the firstbreaker opens to protect the line, and this can be accomplished by thecontrol system. However, it is particularly important that such otherbreakers are not improperly opened when no dangerous conditions occur onthe line by faulty operation of the control system. Accordingly, it isimportant both that the system is fast and reliable to provide thedesired control, and also that it does not through faulty operationprovide controls which are not desired.

It is therefore an object of the invention to provide an improved fastacting control system operated by voice frequency carrier signals.

A further object of the invention is to provide a system for directlycontrolling equipment including a receiver which is blocked when thecontrol signals are not sufiiciently strong to be reliable.

Another object of the invention is to provide an improved receiver forfast acting control systems operating from voice frequency multiplexsignals wherein controlling action is prevented when the relativestrength of the received signals with respect to the noise is notsufficicnt to provide reliable operation.

A feature of the invention is the provision of a fast acting controlsystem operating from voice frequency multiplex signals with filtermeans for selecting the signal and noise, and differential combiningmeans for producing a voltage for blocking the control action when thesignal is not sutficiently greater than noise to provide reliablecontrol action, with the blocking circuit operating faster than thecontrol system to prevent undesired control.

Another feature of the invention is the provision of a blocking circuitwhich samples control signals and noise received from a multiplexchannel and includes a feedback circuit controlled by the strength ofthe signal for simultaneously holding the signal and noise withinpredetermined levels so that control action responds to theinstantaneous relative strengths of signal and noise and is effectiveover a wide range of signal strengths.

A further feature of the invention is the provision of a blockingcircuit for a relay control system wherein the relay actuating circuitincludes an electron discharge valve which is instantaneously renderednon-conducting when the blocking signal is produced so that the relaycircuit is broken before the relay can be operated, to thereby preventrelay operation by undesired signals.

Further objects, features and the attending advantages of the inventionwill be apparent from a consideration of the following description takenin connection with the accompanying drawings, in which:

Fig. 1 is a block diagram of a control system receiver in accordancewith the invention;

Fig. 2 is a chart showing the channel distribution in the voicefrequency multiplex system;

Fig. 3 is a chart illustrating operation of the system; and

Fig. 4 is a circuit diagram of the receiver in accordance with theinvention.

In practicing the invention there is provided a receiver for the controlsystem for controlling equipment in response to voice frequency carriersignals. In order to provide high speed operation, frequency shiftoperation with channel widths of 300 cycles is used so that four voicefrequency carriers and two noise channels are provided on acommunication channel extending from 300 to 3600 cycles. Such a channelmay be provided by a wire line or by one channel of a microwavecommunication system. As the signals may be used to directly controlequipment, it is desired to block the control if the signal on thechannel is not sufficiently greater than the noise on the channel toprovide reliable operation. The signals of the various channels areselected by highly selective filters and are applied to translatingchannels which control relays in accordance with the signals received.Each translating channel includes a discriminator which responds to thefrequency shift signals to provide a control voltage which may operate arelay. A blocking circuit prevents the control operation when the signalreceived is not sufficiently greater than noise, and includes a signalchannel coupled to the signal selecting filter, and a noise channelincluding a filter for selecting a frequency band on which no signalsare transmitted. The signal and noise channels each include an amplifierand a detector for producing a voltage in accordance with the signalstherein. The voltages from the signal and noise channels are combineddifferentially to provide a direct current voltage utilized to block therelay operation. A feedback circuit is provided to hold the outputs ofthe signal and noise channels within desired operating range and appliesa portion of the output of the detector of the control channel to theamplifiers of the signal and noise channels to simultaneously controlthe gain of these two amplifiers. The blocking operation is provided byan electron discharge valve in series with the control relay whichoperates instantaneously in response to the blocking voltage to open therelay circuit before the relay can operate by substandard signals. Analarm circuit may be provided for producing an alarm when the blockingvoltage is present.

Referring now to the drawings, in Fig. 1 there is illustrated in blockdiagram form the receiving system in accordance with the invention. Line10 represents a voice channel over which a plurality of differentsignals are applied by use of voice frequency carriers. Fig. 2illustrates the voice frequency carrier channel distribution which maybe used wherein relatively wide band channels are provided. Fourchannels are provided extending from 900 to 1200 cycles, 1500 to 1800cycles, 2100 to 2400 cycles and 2700 to 3000 cycles, respectively. Aboveand below the four signal channels are two noise channels, one between300 and 640 cycles and the other between 3300 and 3600 cycles. Thesenoise channels are used in a manner to be described. The lower noisechannel is widened somewhat to provide more representative noise pickup.

Referring again to Fig. l the desired carrier is derived from the lineby signal filter 11. Signals received by the filter are applied toamplifier 12 and then to limiter 13. As previously stated, the signalsmay be frequency shift signals, that is, a carrier of a particularfrequency may be normally transmitted and then the frequency of thecarrier shifted when a control operation is desired. The frequency shiftsignal is converted to a direct current control signal by discriminator14. A control signal from discriminator 14 is applied to a relay controlcircuit '15 which controls the receiving relay 16.

In many applications the receiving relay may be directly connected to anitem to be controlled such as a circuit breaker. Accordingly, when thevoice frequency carrier selected by the filter 11 is shifted infrequency, the discriminator 14 will produce a control voltage which isapplied to the relay control circuit 15 to operate the relay 16. It isparticularly important in many applica tions that the relay control notbe actuated by noise which may appear on the communication channel 10,as undesired operation of the relay 16 may cause improper operation ofthe equipment controlled thereby to produce serious damage.

To prevent improper operation of the relay 16, a blocking controlcircuit is provided including a signal channel and a noise channel. Thesignal channel includes a signal amplifier 20 which amplifies the signalselected by filter 11. The amplified signal is applied to detector 21which produces a voltage in accordance with the strength of the signalreceived. The noise channel. includes a noise filter 25 connected to theline 10 and adjusted to select a frequency at which no signal isapplied. The noise filter may be tuned to the low noise channel or thehigh noise channel as shown in Fig. 2. Noise selected by the filter 25is applied to noise amplifier 26 and from the amplifier to detector 27.The outputs of the detectors 21 and 27 are differentially combined andapplied to amplifier 28. This combined voltage is applied through theamplifier 28 to the receiver blocking control 29 to disable the relay16. As shown by the dotted connection 30, a portion of the output of thedetector 21 is fed back to the amplifiers 20 and 26 to control the gainthereof so that the detector outputs fall within a desired range. Thisresults in accurate comparison of the signal with noise for a wide rangeof signal levels.

Connected to the output of the direct current amplifier 28 is an alarmcontrol 31 which operates an alarm relay 32. The alarm relay 32indicates when the blocking control is in operation to show that thenoise is of a suflicient level to interfere with proper operation of thecontrol. This indicates to the operator that the control is notoperating and indicates faulty operation of the entire control system.

As previously stated, a plurality of channels, for example 4, may beapplied to the line 10. As shown'in Fig. 1, additional signal filters 35may be provided which select different channels than that selected bythe filter 11. The same noise output from filter 25 may be used in theblocking control for a plurality of signal channels. Accordingly, aterminal 36 is provided from the output of noise filter 25 which may beused for blocking the operation of receivers utilizing signals fromother channels derived by the signal filter 35.

Reference is now made to Fig. 3 which illustrates the operation of thesystem which has been described. Chart A shows the operation of thetransmitter of a particular channel. At the beginning of the time,marked off, the normal frequency is transmitted. During the shaded part,marked on, the frequency is shifted to produce a control. Chart B showsthe signal to noise ratio of the voice frequency carrier, with thedotted line b showing the signal to noise ratio at which blocking actiontakes place. As the signal to noise ratio of chart B never falls to lineb, no blocking action results as shown by chart C. Chart D shows theoperation of the receiver relay showing that the relay is unoperated atthe beginning and is later closed after the frequency has been shifted.It will be noted that there is a delay of the order of 10 millisecondsfrom the frequency shift to the operation of the relay control, which isthe delay produced in the receiver by the discriminator and the carrierfilter as will be set forth in more detail.

Considering now the action from high noise, chart E of Fig. 3 againshows the operation of the transmitter and the frequency is not shiftedfor the entire duration of the chart. Chart F again shows the signal tonoise ratio at the receiver, and it will be noted that at the point athe signal to noise ratio falls below the dotted line b. Chart G showsthe receiver blocking and shows that at point 0, shortly after point a,the receiver is blocked. As will be explained, the receiver is blockedfor the duration of the time that the signal to noise ratio falls belowthe dotted line and for an additional period of approximatelymilliseconds. Chart H shows the operation of the receiver and indicatesthat the receiver would have operated at the point a had there been noblocking protection at point c. It is noted that point a occurs afterpoint c so that the receiver is blocked before the tripping action cantake place. This is accomplished by design of the blocking circuit sothat the time delay therethrough is less than the delay through thesignal channel.

Referring now to Fig. 4, the more detailed operation of the receiversystem will be apparent from this circuit diagram. The signal selectedby the filter 11 is applied to potentiometer 41 which controls the levelof the signal channel. This signal is amplified by the amplifier 12 andlimited in the stage 13. The frequency shift is translated to a directcurrent control signal by discriminator 14. The voice frequency carrieris removed by filter .1,which applies the control voltage to theparallel connected triodes of the stage .15 to render the sameconducting. The anodes of the triodes are connected to the relay 16.

The signal of potentiometer it? is also applied to a voltage dividerincluding resistors 42 and 43 from which a portion of the signal isapplied to the amplifier 20 of the control channel. The amplifierincludes the pentode stage 44 and a triode stage 39 which feeds thedetector 21. The output of the detector 21 is applied to a load circuitincluding resistors 45 and 46 connected in series and shunted bycondenser 47. A negative voltage is developed by diode 21 acrossresistors 45 and 46 which is a measure of the strength of the signal inthe channel being used. That is, the voltage across resistors 45 and 46is negative at the plate of diode 21 with respect to ground.

The noise signal selected by the filter 25 is applied to potentiometer50, with a portion of the signal from potentiometer being applied toamplifier 26. This amplifier is identical to amplifier 2t and includes afirst pentode stage 51 and triode stage 52. The output of the amplifieris applied to detector 27 which produces a positive voltage across loadresistor 53 which is shunted by a condenser 54. That is, the voltageacross resistor 53 is positive at the cathode of diode 27 with respectto ground. This voltage is a measure of the strength of the noiseselected by filter 25 which will be generally the same as the noise inthe channel selected by filter 11.

As previously stated the diode 21 is connected so that the voltageacross resistors 45 and 46 is negative with respect to ground and thediode 27 is connected so that the voltage appearing across resistor 53is positive with respect to ground. These voltages are combined throughisolating resistors 49 and 55 and applied through resistor 56 to thedirect current amplifier 28. If the combined voltage is more negativethan a predetermined value, for

example, 2 volts, the direct current amplifier 28 is driven to cutoff;Therefore the voltage at the plate is a high positive voltage which issupplied to the grids of the triodes of the blocking stage 29 so thatthese triodes conduct. The relay control triodes 15 are normally veryslightly conducting but not sufficiently conducting to operate the relay16. However, when the frequency is shifted the positive discriminatoroutput causes the control tubes 15 to conduct. When the tubes 15 and 29are both conducting, the relay 16 will operate to close the circuitbetween terminals 57 and 58 which may be connected to provide control ofany desired equipment.

If the received signal level is reduced With respect to the noise, thenegative voltage from the detector 21 will not exceed the positivevoltage from the detector 27 by the required amount to cut off tube 28.Tube 23 will then conduct to hold the plate voltage low and cut off thetriodes of the blocking stage 29. This will prevent opera tion of therelay 16 even though a positive potential is applied to the grids of therelay control tube 15. The input to the noise amplifier 26 controlled bypotentiometer 50 may be adjusted so that the voltage from the noisechannel has a predetermined relation to the voltage derived by thesignal channel to give the desired protection. For example, it may bedesired to let the system operate only when the signal level is 20decibels above the noise level. Adjustment of potentiometer 50 may beset to provide this relationship.

In order to make the system give the same protection for signals ofdifferent strengths, a feedback circuit is provided for controlling theamplifier stages 44 and 51. This feedback circuit is connected from theintermediate point between resistors 45 and 46 to derive a portion ofthe negative voltage produced by the detector 21. The feedback voltageis filtered by condenser 60, resistor 61 and condenser 62 and appliedthrough resistor 63 to the grid of tube 44 and through resistor 64 tothe grid of tube 51. The same feedback bias is applied to both tubes sothat as the gain of tube 44 is reduced to reduce the level of thesignal, the gain of tube 51 is simultaneously reduced to reduce the gainof the noise. By using the same feedback voltage for both amplifiers,the gains are simultaneously controlled whereas the use of a separatefeedback voltage for each amplifier from its respective output mightresult in the gain of one amplifier being reduced before the gain of theother so that the differential output does not represent the relativeinstantaneous levels. The feedback holds the detected voltages at therectifiers 21 and 27 within a predetermined range even though thesignal-s received may vary through a much wider range. This makes therelative differential control operate for substantially the samedifference in signal and noise levels through a wide range. This is tobe desired since the reliability of the receiving system depends uponthe relative levels rather than the absolute levels.

As previously mentioned, the operation of the relay 16 is delayed withrespect to the frequency shift of the voice frequency carrier appliedthrough the filter 11. This delay results in part from the tunedcircuits of the discriminator 14 and in part because of the carrierfilter 41. As it is desired to have very fast acting control, the delayis held to a minimum. Accordingly, the blocking control must operatevery fast to be faster than the control through the signal channel inorder to prevent false operation by noise pulses. To accomplish this theamplifiers and detector of the signal and noise channels must have aminimum time delay so that the blocking action takes effect faster thanthe control action through the signal channel. The use of an electrontube 29 to provide blocking instead of a relay further speed up theblocking action'since the tube will open the circuit of the controlrelay 16 instantaneously before this relay can operate.

It is necessary to hold-the system blocked for a predetermined timeafter each blocking action since noise pulses in the control channel aredelayed and may cause operation of the control tube after the noise haspassed through the blocking circuit. This holding action is accomplishedby the use of condenser 59 connected to the plate of tube 28. The tube28 is normally blocked so that the plate voltage is high and thecondenser 59 is charged to this voltage. However, when the noiseincreases with respect to the signal to the point that the tube 28 isrendered conducting, the condenser 59 will discharge rapidly. That is,the condenser is discharged during the blocking of the system. How ever,when the differential voltage which produces the blocking action ceases,the voltage across condenser 59 will remain low until the condenser cancharge from the B plus potential. This delays the conductivity of thetubes 29 for a predetermined time.

As previously stated an alarm relay is provided for indicating when thesystem is blocked because the noise level is so high with respect to thesignal level that control by the signal will not be reliable. The tube31 of the alarm relay has its grid connected to the plate of tube 28,and since the plate of the tube 28 is normally at a high potential, thetube 31 will normally conduct to actuate the relay 32. However, when thepotential of the plate of triode 28 drops because of increased noise,the tube 31 will cease to conduct and the relay 32 will drop out. Thiscauses the contact 66 thereof to move to a different position to closethe circuit through terminals 67 and 68 to energize an alarm circuit.

It is therefore seen that the receiving system disclosed providesreliable fast acting control. The system may be used to directly controlequipment since it is blocked when the signal falls to a level at whichthe control may be unreliable so that equipment is not improperlyoperated thereby. The overall system is relatively simple with a singlerelay providing the control action and the disabling thereof.

We claim:

1. *In a control system for operation by wide band control signalstransmitted by a carrier wave, and in which the control is reliable onlywhen the carrier wave exceeds the accompanying noise signals by apredetermined amount, receiving apparatus for providing controllingaction only when the strength of the carrier wave at the receivingapparatus exceeds the accompanying noise by the predetermined amount,said apparatus including in combination, first selecting means forselecting the frequency band including the carrier wave, translatingmeans connected to said selecting means for deriving the control signalsfrom the carrier wave, said translating means including a predeterminedtime delay, relay means connected to said translating means and operatedby control signals derived thereby for providing controlling action,first amplifier means connected to said first selecting means, firstrectifier means connected to said first amplifier means for producing afirst voltage varying with the strength of the wave applied to saidfirst rectifier means, second selecting means for selecting a fre quencyband including only noise, second amplifier means having the same gainas said first amplifier means connected to said second selecting means,a feedback circuit connected from said first rectifier means to saidfirst and second amplifier means for controlling the gains thereof tohold the output of said first rectifier means within predeterminedlimits, said feedback circuit controlling the gains of said first andsecond amplifier means so that said gains remain the same, secondrectifier means connected to said second amplifier means for producing asecond voltage varying with the strength of the noise from said secondamplifier means, and means connected to said relay means and responsiveto said first and second voltages for rendering said relay meansinoperative when said first voltage fails to exceed said second voltageby a fixed amount, the delay in said first and second amplifier meansand said first and second rectifier means being less than the delaythrough said translating means.

so that said relay means is rendered inoperative before control signalsare applied thereto.

2. In a control system for opera-tionby wide band control signalstransmitted by a carrier wave, and in which the control is reliable onlywhen the carrier wave exceeds the accompanying noise signals by apredetermined amount, receiving apparatus for providing controlling action only When the strength of the carrier wave at the receivingapparatus exceeds the accompanying noise by the predetermined amount,said apparatus including in combination, first selecting means forselecting the frequency band including the carrier wave, translatingmeans connected to said selecting means for deriving the control signalsfrom the carrier wave, and relay means connected to said translatingmeans and operated by the signals derived thereby for providingcontrolling action, first amplifier means connected to said firstselecting means, first detecting means connected to said first amplifiermeans for producing a first voltage varying with the strength of thewave applied to said first detecting means, second selecting means forselecting a frequency band including noise, second amplifier meansconnected to said second selecting means, second detecting meansconnected to said second amplifier means for producing a second voltagevarying with the strength of the noise from said second amplifier means,a feedback circuit connected from said first detecting means to saidfirst and second amplifier means for controlling the gains thereof tohold the outputs of said firstand second detecting means withinpredetermined limits, and means connected to said relay means andresponsive to said first and second voltages for rendering said relaymeans inoperative when said first voltage fails to exceed said secondvoltage by a fixed amount.

3. In a controlsystem for operation by wide band control signalstransmitted by a carrier wave, and in which the control is reliable onlywhen the carrier wave exceeds the accompanying noise signals by apredetermined amount, apparatus for providing; controlling action at areceiver only when the strength of the carrier wave at the receiverexceeds theaccompanying noise by the predetermined amount, saidapparatus including in combination, relay means adapted to be operatedby the receiver for providing controlling action, first selecting meansfor selecting the frequency band including the carrier wave, firstamplifier means connected to said first selecting means for amplifyingthe carrier wave, first detecting means connected to said firstamplifier means for producing a first voltage varying with the strengthof the amplified wave from said first amplifier means, second selectingmeans for selecting'a frequency band containing noise, second amplifiermeans connected to said second selecting means, second detecting meansconnected to said second amplifier means for producing a second voltagevarying with the strength of the noise from said second amplifier means,:a feedback circuit connected from said first detecting means to saidfirst and second amplifier means for controlling the" gains thereof tohold the output of said first detecting means within predeterminedlimits and to simultaneously control the gain of said second amplifiermeans, and means connected to said relay means and responsive to saidfirst and second voltages for rendering said relay means inoperativewhen said first voltage fails to exceed said second voltage by a fixedamount.

4. In a control system for operation by wide band control signalstransmitted by a carrier wave, and in which the control is reliable onlywhen the carrier wave exceeds the accompanying noise signals by apredetermined amount, receiving apparatus for providing controllingaction only when the strength of the carrier wave at the receivingapparatus exceeds the accompanying noise by the predetermined amount,said apparatus including in combination, first selecting means forselecting the frequency band including the carrier wave, translatingmeans connected to said selecting means for deriving the control signalsfrom the carrier wave, relay means connected to said translating meansand operated by the signals derived thereby for providing controllingaction, first amplifier means connected to said first selecting means,first rectifier means connected to said first amplifier means forproducing a first voltage varying with the strength of the wave fromsaid first amplifier means, second selecting means for selecting afrequency band including only noise, second amplifier means connected tosaid second selecting means, second rectifier means connected to saidsecond amplifier means for producing a second voltage varying with thestrength of the noise "from said second amplifier means, a feedbackcircuit connected from said first rectifier means to said first andsecond amplifier means for controlling the gains thereof to hold theoutputs of said first and second rectifier means within predeterminedlimits, control means responsive to said first and second voltagesactuated when said first voltage fails to exceed said second voltage bya fixed amount, said control means when actuated rendering said relaymeans inoperative, and alarm means connected to said control means forindicating activation of said control means.

In a control system for operation by wide band control signalstransmitted by a carrier Wave, and in which the control is reliable onlywhen the carrier wave exceeds the accompanying noise signals by apredetermined amount, receiving apparatns for providing controllingaction only when the strength of the carrier wave at the receivingapparatus exceeds the accompanying noise by the predetermined amount,said apparatus including in combination, first selecting means forselecting the frequency band including the carrier wave, translatingmeans connected to said selecting means for deriving the control signalsfrom the carrier wave, first control means connected to said translatingmeans and actuated by the signals derived thereby, first amplifier meansconnected to said first selecting means, first rectifier means connectedto said first amplifier means for producin" a first voltage varying withthe strength of the wave from said first amplifier means, secondselecting means for selecting a frequency band including noise, secondamplifier means connected to said second selecting means, secondrectifier means connected to said second amplifier means for producing asecond voltage varying with the strength of the: noise from said secondamplifier means, a feedback circuit connected from said first rectifiermeans to said first and second amplifier means for controlling the gainsthereof to hold the outputs of said first and second rectifier meanswithin predetermined limits, second control means responsive to saidfirst and second voltages and actuated when said first voltage fails toexceed said second voltage by a fixed amount, and relay means connectedto said first and second control means, said relay means being operatedin response to actuation of said first control means in the event saidsecond control means is not actuated, with actuation of said secondcontrol means rendering said relay means inoperative.

6. In a control system for operation by wide band control signalstransmitted by a carrier wave, and in which the control is. reliableonly when the carrier wave exceeds the accompanying noise signals by apredetermined amount, receiving apparatus for providing controllingaction only when the strength of the carrier wave at the receivingapparatus exceeds the accompanying noise by the predetermined amount,said apparatus including in combination, first selecting means forselecting the frequency band including the carrier wave, translatingmeans connected to said selecting means for deriving the control signalsfrom the carrier wave, relay means connected to said translating meansand including a blocking portion, said relay means being operated by thesignals derived from said translating means for providing; controllingaction when said blocking portion areal-e is disabled, first amplifiermeans connected to said first selecting means, first rectifier meansconnected to said first amplifier means for producing a first voltagevarying with the strength of the wave from said first amplifier means,second selecting means for selecting a frequency band including onlynoise, second amplifier means connected to said second selecting means,second rectifier means connected to said second amplifier means forproducing a second voltage varying with the strength of the noise fromsaid second amplifier means, a feedback circuit connected from saidfirst rectifier means to said first and second amplifier means forcontrolling the gains thereof to hold the outputs of said first andsecond rectifier means within predetermined limits, and means connectedto said blocking portion of said relay means and responsive to saidfirst and second voltages for actuating said blocking portion andthereby preventing operation of said relay means when said first voltagefails to exceed said second voltage by a fixed amount.

7. In a control system for operation by wide band control signalstransmitted by a carrier wave, and in which the control is reliable onlywhen the carrier wave exceeds the accompanying noise signals by apredetermined amount, receiving apparatus for providing controllingaction only when the strength of the carrier wave at the receivingapparatus exceeds the accompanying noise by the predetermined amount,said receiving apparatus including in combination, first selecting meansfor selecting the frequency band including the carrier wave, translatingmeans connected to said selecting means for deriving the control signalsfrom the carrier wave, a control circuit including relay means connectedin series with an electron device to said translating means, said relaymeans being operated by the signals derived from said translating meansfor providing controlling action when said electron device isconductive, first amplifier means connected to said first selectingmeans, first rectifier means connected to said first amplifier means forproducing a first voltage varying with the strength of the wave fromsaid first amplifier means, second selecting means for selecting afrequency band including only noise, second amplifier means connected tosaid second selecting means, second rectifier means connected to saidsecond amplifier means for producing a second voltage varying with thestrength of the noise from said second amplifier means, a feedbackcircuit connected from said first rectifier means to said first andsecond amplifier means for controlling the gains thereof to hold theoutputs of said first and second rectifier means within predeterminedlimits, and means connected to said electron device of said controlcircuit and responsive to said first and second voltages for renderingsaid electron device non-conducting when said first voltage fails toexceed said second voltage by a fixed amount to thereby preventoperation of said relay means.

8. In a control system operated by control signals transmitted byfrequency shift of a voice frequency carrier wave in a multiplex system,and in which the control is reliable only when the carrier wave exceedsthe noise on the multiplex system by a predetermined amount, receivingapparatus for providing controlling action including in combination,first selecting means connected to the multiplex system for selectingwaves in a channel carrying the control signals, frequency .shifttranslating means connected to said selecting means for deriving thecontrol signals from said selected waves, controlling relay meansconnected to said translating means and operated by the signals derivedthereby, first amplifier means connected to said first selecting meansfirst rectifier means connected to said first amplifier means forproducing a first voltage varying with the strength of the waves appliedthereto, second selecting means connected to the multiplex system forderiving noise from a channel thereof to which no signals are applied,second amplifier means connected to said second selecting means,

a feedback circuit connected from said first rectifier means to saidfirst and second amplifier means for simultaneously controlling the gainthereof to hold the output of said first rectifier means withinpredetermined limits, second rectifier means connected to said secondamplifier means for producing a second voltage varying with the strengthof the noise from said second amplifier means, and means coupled to saidrelay means and responsive to said first and second voltages forrendering said relay means inoperative when said first voltage fails toexceed said second voltage by a fixed amount.

9. In a control system operated by control signals transmitted byfrequency shift of a voice frequency carrier wave in a multiplex systemand in which the control is reliablev only when the carrier wave exceedsthe noise on the multiplex system by a predetermined amount, receivingapparatus for providing controlling action only when the carrier waveexceeds the noise on the multiplex system by a predetermined amount,said receiving apparatus including in combination, first selecting meansconnected to the multiplex system for selecting waves in a channelcarrying the control signals, frequency shift translating meansconnected to said selecting means for deriving the control signals fromsaid selected waves, a control circuit including relay means connectedin series with an electron device to said translating means, saidelectron device normally being conductive to cause operation of saidrelay means by signals derived by said translating means, firstamplifier means connected to said first selecting means, first rectifiermeans connected to said first amplfier means for producing a firstvoltage varying with the strength of the waves applied thereto, secondselecting means connected to the multiplex system for deriving noisefrom a channel thereof to which no signals are applied, second amplifiermeans connected to said second selecting means, second rectifier meansconnected to said second amplifier means for producing a second voltagevarying with the strength of the noise from said second amplifier means,a feedback circuit connected from said first rectifier means to saidfirst and second amplifier means for simultaneously controlling the gainthereof to hold the output of said first and second rectifying meanswithin predetermined liimts, and means coupled to said electron deviceand responsive to said first and second voltages for rendering saidelectron device non-conducting when said first voltage fails to exceedsaid second voltage by a fixed amount to thereby prevent operation ofsaid relay means.

10. In a control system for operation by wide band control signalstransmitted by a carrier wave, and in which the control is reliable onlywhen the carrier Wave exceeds the accompanying noise signals by apredetermined amount, apparatus for providing controlling action at areceiver only when the strength of the received carrier wave exceeds theaccompanying noise by the predetermined amount, said apparatus includingin combination, control means adapted to be operated by the receiver,first selecting means for selecting the frequency band including thecarrier wave, first translating means connected to said first selectingmeans for controlling the level of the carrier wave, first detectingmeans connected to said first translating means for producing a firstvoltage varying with the strength of the wave from said firsttranslating means, second selecting means for selecting a frequency bandcontaining noise, second translating means connected to said secondselecting means, second detecting means connected to said secondtranslating means for producing a second voltage varying with thestrength of the noise from said second translating means, a feedbackcircuit connected from said first detecting means to said first andsecond translating means for controlling the levels therein to hold theoutput of said first detecting means within predetermined limits and tosimultaneously control the gain of said second translating means, andmeans connected to said control means and responsive'to said first andsecond-voltages for rendering said control means inoperative when saidfirstvoltage fails to exceed saidsecond voltage bya'fijc'ed amount.

11. In a control system operated by control signals transmitted'byfrequency-shift of avoicefrequency carrier wave in a multiplex system,and ill Which the control isreliable only when the carrier Waveexceedsthe noise on the multiplex system by a predetermined amount, receivingapparatus for providing controlling action including in combination,first selecting means connected to the multiplex system for selectingWaves in a channel carrying the control signals, frequency shifttranslating means connected to said selecting means for deriving thecontrol signals from said selected Waves, control means connected tosaid translating means and operated by the signals derived thereby,first circuit means connected to said first selecting meansandlincluding first rectifier means for producing a first voltagcvarying with the strength of the selected Waves, second selecting meansconnected to the multiplex system for deriving noise from a channel 12thereof to which n'osignals are applied, second circuit meansconnected'to' said second selecting means and including second rectifier" meansfor producing a second voltage varyingwith'thc'strength of the noise, afeedback circuit connectedfrom said first rectifier means to said firstand second circuit means for simultaneously controling the'levelsthereinto hold the output of said first rectifier means withinpredetermined limits, and means coupled to said control means andresponsive to said first and second voltages for rendering'said controlmeans inoperative whcn said first voltage fails to exceed said secondvoltage by afixed amount.

References Cited in the file of this patent UNlTED STATES PATENTS2,293,869 Vaughn Aug. 25, 1942 '2,307,77l Denton et a1. Ian. 12, 194-32,533,543 Young Dec. 12, 1950 2,679,000 Reynolds May 18, 1954 2,694,142Lading Nov. 9, 1954

